The present invention relates to a permanent magnet member for use as a magnet roll such as a developing roll, a cleaning roll etc. in an electrophotographic apparatus, an electrostatic recording apparatus, etc., and a method of producing such a permanent magnet member.
In conventional electrophotographic apparatus, electrostatic recording apparatus, etc., a permanent magnet member used as a magnet roll such as a developing roll and a cleaning roll generally has a structure as shown in FIG. 5. The permanent magnet member comprises an integral, hollow cylindrical magnet body 1 constituted by a sintered magnet of hard ferrite, and a shaft 2 disposed at a center of the integral, hollow cylindrical magnet body 1. The integral, hollow cylindrical magnet body 1 is concentrically fixed to the shaft 2.
The integral, hollow cylindrical magnet body 1 is provided with a plurality of magnetic poles (not shown) having alternating polarities on an outer surface thereof, which magnetic poles extend axially and are arranged circumferentially at an equal or unequal interval. A pair of flanges 3 and 4 are rotatably mounted on opposite ends of the shaft 2 via bearings 5, 5. A hollow cylindrical sleeve 6 is fixedly mounted between the flanges 3 and 4 so as to surround the integral, hollow cylindrical magnet body 1. Incidentally, the flanges 3 and 4 and the sleeve 6 are made of a non-magnetic material such as aluminum alloys, stainless steel, etc. A reference numeral 7 denotes a seal member disposed between the flange 3 and the shaft 2. The integral, hollow cylindrical magnet body 1 has an outer diameter of 15-60 mm and an axial length of 200-350 mm.
In the permanent magnet member having the above structure, the integral, hollow cylindrical magnet body 1 and the sleeve 6 rotate relative to each other. For instance, the integral, hollow cylindrical magnet body 1 is kept stationary while the sleeve 6 secured to the flanges 3 and 4 are allowed to rotate relative thereto. By this construction, a magnetic developer is attracted onto a surface of the rotating sleeve 6 by a magnetic attraction force of the integral, hollow cylindrical magnet body 1 to form a magnetic brush and conveyed into a developing region for carrying out the development of latent image on an image-bearing member (not shown), or a residual magnetic developer presenting on the image-bearing member after transferring step is absorbed onto the sleeve 6 for carrying out the cleaning of the image-bearing member.
The permanent magnet member described above is for instance produced in the following manner. First, an adequate amount of polyvinyl alcohol (PVA) is added to barium-ferrite particles, mixed by using a kneader, granulated and dried to obtain a starting particulate material. Next, the starting particulate material is charged into an envelope formed of a rubber or plastic thin film and including a center core rod. The envelope charged with the starting particulate material is immersed in a liquid such as oil, glycerin, water, etc. to apply a fluid pressure therearound so that the starting particulate material is subjected to a pressure-forming (hydrostatic molding or rubber pressing) to produce the integral, hollow cylindrical magnet body 1.
The separately produced shaft designated by the reference numeral 2 in FIG. 5, is inserted into a center bore of the integral, hollow cylindrical magnet body 1, bonded thereto by an adhesive and machined to form the permanent magnet member with a plurality of magnetic poles extending axially and arranged circumferentially on an outer surface thereof.
However, such a conventional production method of the permanent magnet member requires a relatively complicated procedures for producing the integral, hollow cylindrical magnet body 1 from the starting particulate material. In addition, mounting of the shaft 2 into the bore of the integral, hollow cylindrical magnet body 1 also requires complicated procedures in which a gap generated due to a slack fit therebetween is completely filled with adhesive. To this end, it is necessary to remove an excess adhesive flown out from the gap when the shaft 2 is inserted into the integral, hollow cylindrical magnet body 1. Furthermore, the adhesive applied must be subjected to heat treatment for hardening. These procedures lead to an increase in manufacturing steps and manufacturing costs.
On the other hand, a so-called bonded magnet composed primarily of ferrite particles and thermoplastic resin material has been widely used to form the permanent magnet member. In this case, the shaft 2 is held in place within an injection mold. The ferrite particles is poured into the mold together with a heated melt of the thermoplastic resin material to form the integral, hollow cylindrical magnet body 1 around the shaft 2. After subjected to cooling and solidifying processes, the bonded magnet thus shaped is removed from the injection mold.
Incidentally, to produce the anisotropic bonded magnet having improved magnetic properties, there has been proposed a method in which a magnetic-field generating units are disposed in the injection mold. Such an anisotropic bonded magnet is advantageous because the permanent magnet member formed therefrom has a low weight and can be produced by a relatively small number of manufacturing steps. However, the molding procedure therefor requires a complicated mold. Further, when a large number of magnetic poles should be produced on the permanent magnet member, especially those having as small diameter as 20 mm or less, a corresponding large number of the magnetic-field generating units must be disposed in the mold in extremely close relation to each other. As a result, there has been problem that desired magnetization pattern of magnetic poles cannot be produced on the bonded magnet. In addition, the injection mold equipped with a large number of the magnetic-field generating units cannot substantially be manufactured, and even if possible, it would be extremely expensive.
In recent years, there have been an increased demand to produce this kind of a permanent magnet member with reduced size and enhanced magnetic properties, and at low costs. However, the conventional permanent magnets can not satisfactorily meet these requirements.